1. Field of the Invention
The present invention relates generally to new compositions for glass-ceramic products and specifically to new glass-ceramic compositions particularly suited for the fabrication of dental constructs such as artificial teeth.
2. Description of the Prior Art
The use of glass-ceramic materials for the fabrication of dental constructs such as dental appliances, e.g., artificial denture plates, bridges and orthodontic brackets, and prosthetic devices such as inlays, onlays partial or full dentures, crowns, and other restorations has been described, for example, in U.S. Pat. No. 4,431,420. As noted in that patent, all such dental constructs should be (1) inert in the oral environment, (2) able to resist the forces of mastication, (3) capable of being shaped to a desired anatomical configuration, and (4) exhibit the aesthetic qualities of natural teeth.
Glass-ceramic materials offer unique advantages for dental applications of the kind described. Because such materials are supplied initially as glasses, they can be formed into desired product shapes by any of the various shaping techniques known to the glass technologist. Further, the glass shapes can be readily converted to highly crystalline bodies without change of shape by a suitable heat treatment, and thereafter exhibit the desirable physical and chemical characteristics of the predominant crystal phase developed on heating. Such properties may include very high abraded strength, chemical inertness, desirable thermal properties and good visual appearance.
U.S. Pat. No. 2,920,971 contains a general description of the basic principles of glass-ceramic technology and may be referred to for a further description of these materials. U.S. Pat. No. 3,839,055 describes a family of glass-ceramic materials identified as tetrasilicic fluormica glass-ceramics which are characterized by good strength and translucency and are therefore particularly well suited to the manufacture of dental constructs. European Patent EP No. 0083828 indicates a preferred range of tetrasilicic fluormica base compositions for dental applications. Those preferred base compositions, if defined according to current practice in weight percent on the oxide basis (except for fluorine which is reported on an elemental basis), consist essentially of about 45-70% SiO.sub.2, 13-30% MgO, 5-20% K.sub.2 O, and 4-9% F. The addition of up to 2% Al.sub.2 O.sub.3 and up to 7% ZrO.sub.2 to these base compositions was found beneficial to stain resistance in the crystallized glass-ceramic material.
In forming dental constructs from glass-ceramic compositions such as above described, it is frequently desirable to apply to the construct, after thermal conversion to a tetrasilic fluormica body of the desired shape, a final outer layer in the form of an enamel or glaze. This outer layer (hereinafter `glaze layer`) which can be composed of sintered glass, porcelain or the like, can be helpful in controlling the final color and/or translucency of the construct without undesirably modifying its physical properties.
In attempting to develop procedures for applying porcelain glaze layers to portions of the exterior surfaces of dental constructs formed from certain tetrasilicic fluormica glass-ceramics (hereinafter sometimes `mica glass-ceramics`) difficulty had been encountered in consistently achieving a smooth, non-porous and defect-free glaze layer on the outer surface of those constructs formed from high-crystal-content glass-ceramic formulations. Frequently, what was instead achieved was a rough or porous surface film on the outer surface of the porcelain glaze. Further investigation revealed that the phenomenon of surface roughening was not limited to glazed constructs, but could be induced in unglazed bodies as well, simply by processing them through the standard glaze firing cycle. Obviously, surface porosity of any kind would be unacceptable in a dental construct for reasons of aesthetics and hygiene. A solution to the problem of poor glaze layer surface quality in glazed and unglazed mica glass-ceramic products was therefore needed.